The polyaniline (PAn) polymer was synthesized by a chemical method using aniline (AN) as monomer, ammonium persulfate (APS) as initiator, and dodecylbenzene sulfonic acid (DBSA) as dopant at 0 degrees C. Conducting polymer films were prepared on the alumina substrate with an interdigitated electrode by using the dip coating method. These films were soaked in methanol solvent for 1h at room temperature to remove excessive DBSA and then were heated at 70 degrees C for 4h in N-2. The detecting materials having a liquid phase at one atmospheric were precisely controlled by the system having a mass flow controller (MFC), temperature controller and measuring chamber. The sensitivity was estimated by heating from 70 degrees C. The sensitivity observed in PAn sensor was lower than PAn-DBSA sensor at 1000 ppm methanol vapors under N-2. because the additional thermal doping effect was resulted from increasing molecular motion in polymer chain to 70 degrees C but DBSA used as dopant was decomposed by heating over 120 degrees C. It caused the melting point of DBSA to be lower than 120 degrees C. So the thermal properties of PAn and PAn-DBSA were investigated by DSC (differential scanning calorimetry). The DSC was scanned with a heating rate of 10 degrees C/Min. The PAn sample did not show the melting peak but PAn-DBSA sample showed the melting peak at 190 degrees C. From this result, the new crystalline-region was evaluated in PAn-DBSA and increased of crystallinity of PAn polymer. And FE-SEM studies showed that the presence of the dopant strongly influenced the morphology of the polymer.